1. Field of the Invention
The field of the invention is that of high-density mass memories or mass storage units that are essential for high-performance computation systems and for data storage systems.
2. Description of the Prior Art
At present, increasingly miniaturized packages containing one or more chips have been developed for these applications and enable the amount of space occupied on an interconnection card to be reduced. The fact of bringing together the memories and the control circuits has the further effect of improving the electrical performance characteristics and, in particular, of reducing the propagation time of the signals.
Thus, rather than organizing the chips side by side on a plane interconnection substrate, it has already been proposed to stack a number of bare chips, one on top of the other, in order to further increase the integration density and obtain optimum interconnection lengths. The memory circuits are well suited to this type of assembly for they dissipate little power and have a relatively small number of inputs/outputs. Most of them can be connected to signal buses common to many chips. Assemblies of this kind are presently being made by Texas Instruments, Irvine Sensors or again THOMSON-CSF (see C-VAL, 9th European ISHM Conference, Nice, 1993, Conference Report, page 304 and R. T. CROWLEY and E. J. VARDAMAN in MCM'94 Proceedings). A factor of the order of 100 in terms of volume density may thus be obtained as compared with conventional packages.
The technologies used initially require a method to reconfigure the inputs/outputs of the chips on one or more sides in order to obtain access thereto once the stack is made. This may be done on the silicon wafer or else on the cut-out chip. FIG. 1 shows one of these methods where a thin printed circuit 1 is mounted by bonding or wiring on the surface of the chip 2. This printed circuit has the role of mounting the inputs/outputs located in the plane of the chip (for example on its two small sides) by means of tracks located above this plane and reaching beyond the edge of the chip (for example on one of its large sides). The chip can be bonded to the printed circuit by means of an adhesive element 3. The wiring is done with wires 4 inserted into windows 5 made in the printed circuit 1.
After reconfiguration in this manner, a number of chips (for example 8) are stacked and held together by means of adhesive films or films of encapsulating liquid resin. FIG. 2 shows a stack of reconfigured chips of this kind. In the case of the example referred to, the thin printed circuits also have centering holes 6 located beyond the border of the chip. These centering holes 6 can be used to position the chips with respect to one another during the stacking. The chips and the printed circuits are superimposed by means of positioning rods 7 and shims used to set the pitch of a stack. This assembly is then coated with a thermo-hardening liquid resin that will fill all the interstices and hold the unit mechanically.
The three-dimensional block 8 thus made is prepared on the face providing access to the inputs/outputs of the chip, to enable these chips to be connected. For this purpose, the resin may be machined on the side containing the tracks that go beyond the edge of the chips so as to obtain a plane resin face comprising metal zones corresponding to the sections of said tracks as illustrated in FIG. 3.
These inputs/outputs are then interconnected either by metallization and etching to form conductive tracks or by being mounted, by soldering, on an interconnection substrate. FIG. 4 illustrates the mounting of a memory block 8, comprising several chips, on an interconnection substrate 9. Through this mounting operation, it is generally possible to increase the pitch between contiguous inputs/outputs to give inputs/outputs that can easily be attached to a standard printed circuit 11 comprising an interconnection 12 used to connect the memory block 8 to a control circuit 10 also mounted on the printed circuit 11.
The connection of a block 8 to the substrate 9 may typically be made by wiring, bonding with conductive resin, lugs or metal beads.
The unit formed by the block 8 and the control circuit 10 mounted on the printed circuit 11 gives a functional mass memory.